What Does Mediastinal Lymph Node Mean?

The mediastinum is the central compartment of the chest, between the two lungs. It holds the heart. It holds the great vessels, the esophagus and the windpipe. It also holds a large number of lymph nodes.

Those nodes drain the lungs. So they show up on almost every chest CT. They get described whether or not anything is wrong with them. A report naming them is doing its job, not raising an alarm.

In lung cancer staging, position matters more than size. The N categories are defined by location:

• N1. Nodes on the same side, alongside the airways or at the hilum, including spread by direct extension.
• N2. Metastasis in mediastinal nodes on the same side as the tumor, or in subcarinal nodes — those beneath the point where the windpipe divides.
• N3. Metastasis in mediastinal nodes on the opposite side, in the opposite hilum, or in scalene or supraclavicular nodes on either side.

That distinction carries real weight. NCI notes that unresectable N2 to N3 disease is handled differently from earlier disease. It also notes that some people with T3 or N2 disease can still be treated with surgery, plus chemotherapy or chemoradiation.

Radiology reports give node measurements, and one number does most of the work.

NCI gives a threshold for clinically operable non-small cell lung cancer. The evidence supports biopsy of any mediastinal node larger than 1 cm in shortest transverse axis on chest CT. It supports biopsy of any node positive on PET as well.

Note which measurement that is. The shortest transverse axis is the narrower width across the node, not its longest dimension. A node described as 14 by 8 mm has not crossed that threshold. The 8 is the number that counts. Reports do not always make that obvious, so it is fair to ask which figure the team is using.

PET is more accurate than CT for staging the mediastinum, and it is still wrong often enough to matter.

A systematic review pulled together 44 studies published between 1994 and 2006. They covered 2,865 evaluable patients. Median prevalence of mediastinal metastasis was 29%. Pooled sensitivity was 74%. Pooled specificity was 85%.

The performance splits sharply by node size:

• Enlarged nodes. Median sensitivity 100%, specificity 78%. PET falsely identifies cancer in about one quarter of patients whose nodes are enlarged for other reasons — usually inflammation or infection.
• Normal-sized nodes. Median sensitivity 82%, specificity 93%. Nearly 20% of patients with normal-sized but malignant nodes had a false-negative PET result.

That is the honest picture. A bright node is not proof. A quiet node is not clearance.

NCI draws two conclusions from those numbers, and both push toward biopsy.

First, a negative PET does not remove the need to biopsy an enlarged node. Second, mediastinoscopy is necessary when the CT and the PET do not agree with each other.

There is also a cost argument that failed. Decision analyses suggested money could be saved by skipping mediastinoscopy when PET was positive. NCI reports that studies rejected this. The number of false-positive results was unacceptably high.

NCI lists three procedures used to obtain tissue samples: bronchoscopy, mediastinoscopy, and anterior mediastinotomy.

Which one is chosen depends on where the node sits. Some stations are reachable through the airway. Others are reached through a small incision at the base of the neck, or alongside the sternum.

At surgery there is a further decision. The Cochrane Collaboration reviewed 11 randomized trials of surgery for early-stage lung cancer, covering 1,910 patients. A pooled analysis of three of those trials found four-year survival was better when resection included complete removal of the mediastinal nodes on the tumor side. The comparison was resection plus sampling only. The hazard ratio was 0.78.

• What is the node's shortest transverse measurement?
• Which side is it on, relative to the tumor?
• Was a PET done, and did it agree with the CT?
• If the two disagree, is mediastinoscopy the next step?
• Which route would a biopsy take?
• Have older scans been pulled for comparison?

• It is not a diagnosis on its own.
• It does not say whether the node is new or unchanged. Comparison with older scans often answers more than any new test.
• It does not distinguish infection or inflammation from cancer.
• It does not replace the full report, or the images themselves.

Mediastinal nodes also come up outside lung cancer. They matter in lymphoma, and in cancers of the esophagus, because the same drainage territory is involved. But the staging rules above are specific to lung cancer. They do not transfer.

If a report uses the words "recommend," "urgent," "critical" or "correlate clinically," that phrasing is aimed at the ordering clinician. Asking what timeline they attach to it is reasonable.

The single most useful thing in many of these reports is not a scan yet to be done. It is a scan already done.

A node that measured the same 12 mm three years ago is a different object from a node that was 6 mm three years ago. Neither number alone tells anyone that. The change does.

So it is worth asking whether prior imaging has been retrieved, including scans done at another hospital for an unrelated reason. Old films are frequently the reason a biopsy turns out to be unnecessary.

Start with imaging tests, then what an enlarged lymph node means on a scan.

• NCI PDQ — Non-Small Cell Lung Cancer Treatment (Health Professional Version)